Electrophotographic imaging system including a halogen treatment step for making background areas transparent



Oct. 27, 1970 J. B. WELLS 3,536,482

ELECTROPHOTOGRAPHIC IMAGING SYSTEM INCLUDING A HALOGEN TREATMENT STEP FOR MAKING BACKGROUND AREAS TRANSPARENT Filed Aug. 29, 1966 I NVENTOR. JOHN B. WELLS A 7' TORNEY United States Patent ELECTROPHOTOGRAPHIC IMAGING SYSTEM IN- CLUDING A HALOGEN TREATMENT STEP FOR MAKING BACKGROUND AREAS TRANSPARENT John B. Wells, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Aug. 29, 1966, Ser. No. 575,638 Int. Cl. G03g 13/22 US. Cl. 96-1 8 Claims ABSTRACT OF THE DISCLOSURE An imaging process in which a photoconductive insulating layer containing a halogen transparentizable Lewis acid sensitized aromatic resin and a halogen transparentizable electrically conductive backing layer is electrostatically developed with electroscopic marking material to form a loosely adhering image on the photoconductive insulating layer and thereafter contacted with a halogen and a liquid which is at least a partial solvent for the photoconductive insulating layer whereby the photoconductive insulating layer and the conductive backing layer are substantially transparentized and the marking material is seated in the photoconductive insulating layer.

This invention relates in general to an imaging system and more specifically to a xerographic type imaging system capable of producing fixed high quality, high resolution images capable of being viewed directly and by use of conventional projection techniques.

In the process of xerography, for example, as disclosed in Carlson Pat. 2,297,691, a xerographic plate comprising a layer of photoconductive insulating material on a conductive backing is given a uniform electric charge over its surface and is then exposed to the subject matter to be reproduced, usually by conventional projection techniques. This exposure discharges the plate areas in accordance with the radiation intensity that reaches them and thereby creates an electrostatic latent image on or in the photoconductive layer. Development of the latent image is effected with an electrostatically charged finely divided material such as electroscopic powder orother electroscopic marking particles that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the latent electrostatic image. Thereafter, the developed image may be fixed or may be transferred to a support surface to which it may be affixed by any suitable means.

Of course, silver photography is still presently employed in many copying and image reproducing applications including the much sought after commercial market termed micro-imaging, also called micro-photography or micro-filming. The chief application of micro-imaging today is in the reproduction of records such as business records and bank checks and so on, and in the reproduc tion of books, newspapers, magazines and other printed matter in a greatly reduced scale for ease in filing, sorting and storing for later retrieval.

Although the micro-imaging field has advanced technologically in recent years with advances in microfilm readers and sorters, the basic process of silver photography has remained the dominant process in the field.

3,536,482 Patented Oct. 27, 1970 Although xerography is a convenient, advantageous and very flexible copying process which has found and is finding its place in many applications outside the ofiice copying field, a number of characteristics of the xerographic process has kept xerography from displacing silver photography as the predominant process in the micro-imaging field.

For example, xerography conventionally employs a conductive substrate which for many applications including micro-imaging should be substantially transparent to allow the resultant image to be viewed by projection techniques. Generally, materials which are electrically conductive enough are not sufficiently transparent and ma terials which are transparent'enough are not as electrically conductive as is preferred.

Also many photoconductive insulating materials for use in xerography are opaque or employ colored sensitizers in a transparent binder which impart color to the binder material thus adversely affecting the transparency of the finally imaged member.

Additionally, it is generally a characteristic of xerography that the marking particle image be fixedly secured to the plate to permit normal handling of the image member. Heat fixing or pressure fixing has been found to have a degrading effect on loose particle image resolution and spraying the image with a transparent coating or overcoating the image with a clear layer adds expense and complexity to the system.

Also, xerographic imaging systems have often lacked the capability of producing images of sufiiciently high resolution, for example, to be suitable for use in micro imaging. For example, the range of resolutions in recently available negative microfilm emulsions is between and 335 l.p./mm. (line pairs per millimeter) as reported by Luther, The Film in Microfilm, Proceedings, The National Microfilm Association (1958). Silver photography is capable of providing micro-images of such resolutions while xerographically produced images, at least those produced by presently available commercial equipment, generally exhibit relatively low resolution. For example, most xerographic ofiice copiers are capable of producing images with resolutions in the area from about 4-10 l.p./mm. which, of course, is entirely suitable for office copying and many other copying applications, but which is not acceptable for micro-imaging and other purposes requiring high resolution.

It is therefore an object of this invention to provide a xerographic imaging system which overcomes the above noted disadvantages and satisfies the above noted needs.

It is another object of this invention to provide a xerographic imaging system capable of producing fixed high quality images.

It is a further object of this invention to provide a xerographic imaging system capable of producing images with substantially transparent background areas.

It is a still further object of this invention to provide a xerographic imaging system capable of producing high resolution images.

It is a still further object of this invention to provide a xerographic imaging system capable of producing images with stability under aging.

It is a still further object of this invention to provide a xerographic imaging system capable of producing directly viewable light-absorbing images which images may also be used as transparencies.

It is a still further object of this invention to provide a xerographic imaging system especially suited for producing micro-images.

The foregoing objects and others are accomplished in accordance with this invention by providing a xerographic imaging system including a xerographic plate comprising a transparentizable conductive layer overcoated with a photoconductive insulating material, said xerographic plate to be processed in accordance with the invention by uniformly charging the plate, exposing the plate to a pattern of activating electromagnetic radiation and developing the latent image all by conventional xerographic techniques, and then treating the plate with the unfixed image loosely adhering thereon with a halogen in at least a partial solvent for the photoconductive material, Treatment in accordance herewith is found to remove color from the photoconductive insulating material to substantially transparentize said transparentizable conductive layer and sufficiently soften and tackify the image bearing surface of the photoconductive insulating layer to cause seating of the electroscopic marking particles in the photoconductive insulating material surface to form a resultant fixed, high resolution, high density xerographic image on a transparent base without transfer to another base. Thus is provided an imaging process suitable for many imaging applications including the field of microimaging. The resultant image may be viewed directly as a light absorbing image or may be used as a transparency utilizing conventional projection techniques.

For a better understanding of the invention as Well as other objects and further features thereof, reference is made to the following detailed disclosure of this invention taken in conjunction with the accompanying figure which is a schematic representation of a xerographic imaging system according to the invention.

Referring to the figure there is illustrated a xerographic plate in the form of a continuous web which passes from supply roller 16 successively through a charging means 18, an exposure means 20, developing apparatus 22, treatment station 24, rinsing means 26 and dryer 28 to take up roll 30.

Plate or web 10 comprises photoconductive insulating layer 14 overlying a transparentizable electrically con duc'tive substrate 12.

Photoconductive insulating layer 14 may comprise any photoconductive insulating material which can be substantially transparentized by treatment herein with a halogen in a solvent for the photoconductive insulating material,

It has been found herein that various charge transfer complexes comprising a suitable aromatic resin sensitized with a suitable Lewis acid are good photoconductors and are transparentizable when treated with a halogen in a suitable solvent for the resin.

Polyvinyl carbazole sensitized with trinitrofluorenone has been found to be a preferred photoconductive insulating material herein because of its high photoconductivity and because when treated with a halogen in a sol vent for the polyvinyl carbazole, for example, toluene, it is found that the yellow color imparted to the polyvinyl carbazole by the trinitrofluorenone is removed to leave a substantially transparent photoconductive insulating layer 14 in background areas. The solvent also softens and sufiiciently tackifies the surface of the photoconductive insulating material to allow the loose electroscopic marking particle image deposited thereon after image development to be seated in the layer to become fixedly secured thereon when the treating solution has been rinsed away and plate 10 dried.

Photoconductive insulating layer 14 may comprise any suitable Lewis acid sensitized aromatic resin. Typical Lewis acids include 2,4,7-trinitro-9-fluorenone; 4,4-bis- (dimethylamino) benzophenone; tetrachlorophthalic anhydride; chloranil; picric acid; benzanthracene-7,l2-dione, 1,3,5-trinitro-benzene and mixtures thereof. Among the resins which form useful charge transfer complexes with suitable Lewis acids are aromatic resins such as polycarbonates, polyurethanes, phenoxy resins, epoxy resins, silicone resins, phenol-formaldehyde resins and mixtures thereof.

Transparentizable conductive layer 12 may comprise any material electrically conductive during the charging and exposing processing steps hereof which may be made transparent during the treatment step by reaction with the halogen carrying solvent. Typical materials which may be converted from an opaque electrically conductive form to a transparent form by reaction with a halogen include copper, cadmium, silver, zinc, aluminum, gold and mixtures thereof. Copper is a preferred material for layer 12 since copper is an excellent electrical conductor and is readily converted into especially transparent metal halide compounds When treated With a halogen. For example, copper when treated with iodine forms transparent cuprous iodide, CuI, which, depending on the thickness of the CuI layer and the wavelength of the incident light, has been found to transmit over of incident light. Lyon Pat. 2,756,165 describes in further detail the coating of copper layers and the conversion of said layers to transparent CuI.

Copper is also a preferred material because copper is a relatively inexpensive and commercially available material and is easily and cheaply formed into thin films for use herein. Also, images formed on a xerographic plate herein with a CuI backing layer are very stable and, to date, indications are that said images possess archival stability.

As the plate 10 in web form passes from supply roll 16 to take up roll 30 it is processed according to the invention herein to provide high resolution and high quality images.

As is well known in the art, generally the first xerographic processing step is to form a latent electrostatic image on the plate 10. This image is illustratively formed by uniformly electrostatically charging the plate by means 18 and then at least partially discharging the plate in light struck areas by exposure means 20. A wide variety of charging systems have been evolved over the years in the art of xerography including vigorously rubbing the layer with a softened material such as a cotton or silk handkerchief or a soft brush or a fur, induction charging, an example of which is described in Walkup Pat. 2,934,649, roll charging as described in Straugham, Mayer, Proc. Nat. Electronics Conf., 13, 959, 962 (1958), depositing charge from a corona discharge device and other techniques. Uniform charging by corona discharge devices which generally can apply either positive or negative charge-producing particles and which come in many shapes and forms for adaptation to many applications generally have been found to be preferred charge applicators. For example, corona discharge devices of the general description and generally operated as disclosed in Vyverberg Pat. 2,836,725 and an embodiment of which is shown as charging means 18 and Walkup Pat. 2,777,957 have been found to be excellent sources of corona useful in the charging of xerographic plates. Also radioactive sources of corona, as described in Dessauer, Mott, Bogdonoff, Photo Eng. 6,250 (1955) as well as other sources of corona are available for use herein.

Other methods of forming a latent image on plate 10 are known in the art and include first forming such a charge pattern on a separate photoconductive insulating layer according to conventional xerographic reproduction techniques and then transferring this charge pattern to layer 14 of plate 10 by bringing the two layers into very close proximity and utilizing breakdown techniques as described, for example, in Carlson Pat. 2,982,647 and Walkup Pats. 2,825,814 and 2,937,943. In addition, charge patterns conforming to selected, shaped, electrodes or combinations of electrodes may be formed on layer 14 by the TESI discharge technique as more fully described in Schwertz Pats. 3,023,731 and 2,919,967 or by techniques described in Walkup Pats. 3,001,848 and 3,001,849 as Well as by electron beam recording techniques, as described in Glenn Pat. 3,113,179.

Preferably, the xerographic plate should be charged when it is at its highest insulating value or when there is an absence of electromagnetic radiation that would make the photoconductive insulating layer 14 pho-toelectrically conductive. To allow the charge to remain on the surface of the layer once deposited there, charging must of course take place in the absence of that wavelength radiation or light to which the particular photoconductive material is sensitive.

After charging, the web moves beneath exposure means whereat an image 3-2 to be reproduced is projected onto the web surface by means of lens 34 desirably operating in conjunction with web exposure mechanism (not shown) both synchronized to the motion of the web. The web exposed to the pattern of activating electromagnetic radiation discharges the plate in light struck areas to leave a latent electrostatic image on the plate corresponding to, for example, dark or image areas of a positive original to be copied, or corresponding to the dark and substantially opaque portions of a negative transparency.

The latent electrostatic image is then rendered visible or developed by developing apparatus 22 by contacting the latent image areas 'with a finely divided marking material that is brought into surface contact with the photoconductive layer and is held thereon electrostatically in a pattern corresponding to the electrostatic latent image. Illustratively, an aqueous development means of the sort described in Gundlach Pat. 3,084,043 is shown as developing apparatus 22.

It is apparent that the resolution of images resulting from the novel system herein is directly dependent on the resolution of the loosely adhering marking particle images that are formed on the free surface of the photo conductive layer after development.

Fluid development systems have been found most suit.- able for developing of images herein because of the extremely high resolutions possible with fluid development. Typical fluid development systems suitable for use herein to provide high resolution images especially suited for micro-imaging purposes are described in Carlson Pats. 2,221,776, 2,551,582, 2,690,394, 2,761,416, 2,928,575; Thompson Pat. 3,064,622; Gundlach Pats. 3,068,115 and 3,084,043; and Metcalfe Pats. 2,907, 67 4, 3,001,888, 3,032,- 432 and 3,078,231.

The hydrocarbon liquid developer compositions described in the Metcalfe patents have been found to be especially preferred for producing high quality, very high resolution images according to the novel system disclosed herein. As reported for example in Metcalfe Pat. 2,907,- 674, resolutions higher than 1,000 l.p./mm. and greater than those available from silver photography at equivalent speeds are possible.

It should be understood that high resolution is a requisite for micro-imaging applications of the novel system herein and generally consists of gravitationally flowing cations where the higher micro-imaging resolutions are not required and thus other suitable development systems in xerography may accordingly be utilized.

For example, the system of cascade development has found extensive commercial acceptance and is suitable herein and generally consists of gravitationally flowing developer material consisting of a two-component material of the type disclosed in Walkup Pat. 2,638,416 over the xerographic plate bearing the latent image. The two components consist of an electroscopic powder termed toner and a granular material called carrier and which by mixing acquire triboelectric charges of opposite polarities. In development, the toner component, usually oppositely charged to the latent image, is deposited on the latent electrostatic image to render that image visible.

Other typical developing systems include magnetic brush development, for example, see Giamo Pat. 2,930,351; skid development, for example, see Mayo Pat. 2,895,847 and others.

After the charging, exposing and developing steps of xerography, the xerographic plate bearing a loosely adhering marking particle image on the surface of the photoconductive insulating layer 14 advances to treatment station 24 where the web is contacted with a mixture of a halogen in a sol-vent for the photoconductive material,

The treating mixture 34 may be applied in a variety of ways including dipping the developed plate into the solvent or rolling or spraying the solvent onto one or both surfaces of the plate as shown in the figure. The purpose and effect of treatment is to transparentize mainly in non-image areas the photoconductive insulating layer and the conductive backing layer and to soften and tackify at least the surface of the photoconductive insulating layer sufliciently to seat the marking particle image therein to fixedly secure the image to the xerographic plate upon drying. For suitable solvents, for example toluene, of the photoconductive insulating material comprising layer 14, it is found that application of the mixture only to the surface of layer 14 is sufficient to transparentize both layer 14 and layer 12 and soften at least the surface of layer 14.

The treating mixture, after application to the plate, is found to leach or bleach out the color in the photoconductive layer 14 for example that is commonly imparted to aromatic resins by Lewis'acid sensitizers, to render layer 14 substantially transparent. The mixture transparentizes conductive backing layer 12 by action of the halogen contained in the mixture to convert the backing layer of metal to a metal halide.

The solvent component of mixture 32 may be any solvent capable of carrying a halogen and capable of at least partially dissolving the aromatic resin comprising layer 14 to substantially transparentize the layer and sufficiently soften and tackify at least the surface thereof to permit seating and fixing of the marking particles.

Toluene has been found to be a preferred solvent for use herein because of its commercial availability and its ability to dissolve and carry iodine and other halogens and its ability to dissolve and soften aromatic type resins including the preferred resin polyvinyl carbazole.

Any suitable solvent or partial solvent for aromatic and similar type resins may be used herein. Typical such solvents are toluene, benzene, cyclohexane, nhexane, phenol, methylethylketone, dimethylsulfoxide, carbon disulfide, carbon tetrachloride and tetrachlorodifluorethane available as Freon 112 from E. I. du Pont de Nemours & Company.

Although any halogen may be used to transparentize a suitable conductive backing layer, treatment with iodine is preferred because iodine is readily soluble in most solvents for aromatic and similar type resins and because the iodides of metals comprising the conductive backing layer are highly transparent.

The conversion of the conductive layer to a transparent form may be accomplished at any suitable time before or after development of the image. For example, the plate 10 may be dipped into a liquid carrying a halogen or exposed to vapors of a halogen after exposure but before development. Preferably, the conversion takes place after the formation of the loose marking particle image as part of a single treatment step which also transparentizes layer 14 and softens and tackifies at least the surface of said layer to fix the image to the plate.

Since background areas are not transparentized by etching or by dissolving portions of material away, there is no undercutting of image areas to make image edges appear ragged and even to remove fine detail which has been associated with etching type processes.

It is noted that during treatment both layers 14 and 12 are found to transparentize initially in non-image, background areas followed by transparentization of layer portions corresponding to image areas. Rinsing the solvent away or applying a reaction stopping agent such as a fatty acid to stop treatment so that transparentization takes place mainly in background areas is a technique for increasing the density and contrast of the resultant image as desired.

After treatment, the web advances to rinsing means 26 whereat any excessive coloration contributed by the halogen may be rinsed away conveniently with the same solvent used in treatment.

The web then advances past dryer 28 and then to take up roll 30 where the fixed, high resolution, substantially transparent background area images are stored for reference and later utilization.

Although xerographic plate 10 has been illustrated and described as a two layer member, for convenience in processing and for ease of manufacture, it is sometimes found desirable to support the plate on a film of transparent support material. Polyester films such as Mylar and Cronar available from E. I. du Pont de Nemours & Company are suitable in this regard.

The following examples further specifically define the present invention with respect to a novel xerographic imaging system especially suitable for micro-imaging applications. The parts and percentages are by weight unless otherwise indicated. The examples below are intended to illustrate various preferred embodiments of the imaging system of this invention.

EXAMPLE I A sheet of Mylar film is coated by vacuum deposition with a layer of copper until the layer has a resistivity of about 10 ohms per square and a transparency of about 30 percent.

A photoconductive insulating layer is formed by dipcoating the copper surface into a solution of about 6% polyvinyl carbazole/trinitrofiuorenone (9:1) in a 126:3 solution of methylene chloride/benzene/toluene. The coating is allowed to dry at room temperature to form a layer of about microns in thickness.

The thus-formed plate is uniformly electrostatically charged to a positive potential of about 600 volts by a corona discharge device as described in Vyverberg Pat. 2,836,725. The uniformly charged plate is exposed to a light-and-shadow image with light intensity in illuminated areas of about 1.5 10" photons per square centimeter. The resultant latent electrostatic image is fluid developed by dipping the plate in a mixture of about 5 parts carbon black of particle size between about 10 and 30 millimicrons and about 40 parts of Duraplex D-65a, alkyd resin available from Rohm & Haas Company pigmented in about 60 parts xylene, the pigment mixture then put in about 1,000 parts kerosense.

The resultant visible image made up of loosely adhering carbon black and resin marking particles is treated by dipping the plate into a solution of about 5% iodine in toluene for about 10 seconds to transparentize the copper layer, remove the yellow color imparted to the polyvinyl carbazole by the trinitrofluorenone and tackify the surface of the polyvinyl carbazole.

The plate is removed from the treating solution, rinsed under flowing toluene and dried yielding a fixed high density, high resolution image with substantially transparent background areas.

EXAMPLE II A second plate is prepared as in Example I except that in place of the copper layer, a thin silver coating is formed. The plate is charged, exposed and developed as in Example I.

The resultant loosely adhering image is treated by dipping the plate into a solution of about 5% bromine in toluene for about 20 seconds to transparentize the silver layer, remove the yellow color from the photoconductive insulating layer and tackify the surface of said layer.

The plate is rinsed and dried as in Example I to yield a fixed high density, high resolution image with substantially transparent background areas.

EXAMPLE III A plate and visible marking particle image are produced as described in Example I.

A solution of about 5% chlorine in toluene is sprayed onto the marking particle surface of the plate and allowed to remain thereon for about 30 seconds to transparentize the copper layer, remove the yellow color from the photoconductive insulating layer and tackify the surface of said layer.

The plate is rinsed and dried as in Example I to yield a fixed, high density, high resolution image with substantially transparent background areas.

Although specific materials have been stated in the above description of preferred embodiments of the novel xerographic imaging system herein, other suitable materials as listed herein may be used with similar results. In addition, other materials may be added to the xerographic plate or variations may be made in the various processing steps to synergize, enhance or otherwise modify the xerographic system described herein. For example, for applications where the resultant image is not to be used as a transparency, it might be found desirable to support xerographic plate 10 on a layer of paper or other suitable materal to provide high contrast relative to image portions. Additionally, for highly abrading uses of the images produced herein such as in the form of rolls of micro-image film to be subject to frequent use in film viewers, it might be found desirable to provide a clear overcoating layer over the image as a further means to resist image abrasion. For normal use the fixing by treatment herein is sufiicient. Also, of course, heating may be employed during the treating step hereof to speed up the treating reaction and/or to aid in tackifying the surface of the photoconductive insulating layer.

It will be understood that various other changes of the details, materials steps and arrangements of materials which have been herein described and illustrated in order to explain the nature of the invention will occur to and may be made by those skilled in the art upon a reading of this disclosure and such changes are intended to be included within the principle and scope of the invention.

What is claimed is:

1. An imaging process comprising the steps of:

(a) providing a xerographic imaging member comprising a photoconductive insulating layer consisting essentially of a Lewis acid sensitized aromatic resin overlying an electrically conductive metallic backing layer wherein both layers are capable of becoming transparent upon exposure to a halogen;

(b) forming an electrostatic latent image on said photoconductive insulating layer;

(c) contacting said photoconductive insulating layer with electroscopic marking material whereby a loosely adhering visible image is formed;

(d) contacting said xerographic imaging member with a halogen and a liquid which is at least a partial solvent for said photoconductive insulating layer whereby said photoconductive insulating layer and said conductive backing layer are substantially transparentized and said marking material is seated in said photoconductive insulating layer.

2. An imaging process according to claim 1 wherein said halogen and said liquid are in mixture.

3. An imaging process according to claim 1 wherein said aromatic resin comprises polyvinyl carbazole and said Lewis acid comprises trinitrofluorenone.

4. An imaging process according toclaim 1 wherein said electrically conductive backing layer comprises a metal selected from the group consisting of copper, cadmium, silver, zinc, aluminum, gold and mixtures thereof.

5. An imaging process according to claim 3 wherein said electrically conductive backing layer comprises a metal selected from the group consisting of copper, cadmium, silver, zinc, aluminum, gold and mixtures thereof.

6. An imaging process according to claim 1 including the steps of rinsing away the treating mixture and drying the resultant image bearing plate to fixedly secure the marking particle image thereto.

7. An imaging process according to claim 1 wherein said plate is supported by a layer of transparent support material.

8. An imaging process according to claim 3 wherein W said halogen comprises iodine and wherein said transparentizable electrically conductive backing layer comprises copper.

References Cited UNITED STATES PATENTS 6/1962 Hoegl et al 961 5/1966 Eastman 961 U.S. Cl. X.R. 96-27; 1l7-17.5 

